In accordance with social demands to energy saving in recent years, silica is compounded so frequently as a filler which allows low heat build-up of a rubber composition for a tire to be consistent with a gripping property of a tire on a wet road surface for the purpose of saving fuel consumption of automobiles.
Silica used tends to cause coagulation of particles themselves by virtue of a hydrogen bond of a silanol group which is a surface functional group, and in order to improve dispersibility of silica in a rubber, a kneading time has to be long. Also, silica causes the defects that a Mooney viscosity of a rubber composition is raised due to its unsatisfactory dispersibility in a rubber and that the rubber composition is inferior in processability such as extrusion. Further, the surface of silica is acidic, and therefore silica involves the defects that it adsorbs thereon basic substances which are used as a vulcanization accelerator to prevent the rubber composition from being sufficiently vulcanized and that the storage modulus of the rubber composition is not enhanced. Accordingly, silica-compounded rubber compositions have so far been desired to be improved in processability and the like.
A rubber composition improved in an electrostatic property which is characterized by compounding 100 parts by weight of a rubber component containing 90 parts by weight or more of a diene base rubber with 30 to 120 parts by weight of a filler containing 40% by weight or more of a white filler such as silica and aluminum hydroxide and 0.2 to 8 parts by weight of a nonionic surfactant or specific phosphoric acid ester has so far been known as technology for improving various properties and the like in a silica-compounded rubber composition (refer to, for example, patent document 1 filed by the present applicants).
However, the purpose of the patent document 1 described above is to reduce a volume resistance value and inhibit generation of static electricity to improve an electrostatic property, and it is neither described, suggested nor indicated therein to improve dispersibility of silica in the rubber composition, to lower a viscosity of an unvulcanized rubber and to improve as well heat build-up of a vulcanized rubber as is the case with the present invention. Further, it is shown in the examples thereof that an addition amount of silica is as small as 50 parts by mass or less at most, and the above patent document 1 is different in a technical concept from the present invention.